The studies aim to characterize in detail the biochemical-genetic regulation of the essential folate-pathways in cultured mammalian, especially human, cells. The activities of 9 of the 15 major folate-pathway enzymes have been detected in crude extracts of cultured skin fibroblasts, peripheral blood lymphoblasts and amniotic fluid cells. Activities of individual folate enzymes and of the pathway as a whole have been shown to be controlled in part by cell growth rate and density, and 5-methyltetrahydrofolate: homocysteine cobalamin methyltransferase activity is controlled in part by the composition of the medium, being derepressed on substitution of homocysteine for methionine. Skin fibroblasts cultured from patients with inborn errors of folate metabolism are being used to understand the biochemical changes in methylenetetrahydrofolate deficiency and dihydrofolate reductase deficiency. Related inborn errors that cause mental retardation and/or neurologic abnormalities are under study in cell culture and include nonketotic hyperglycinemia, dihydropteridine reductase deficiency and the oculocerebralrenal syndrome of Lowe.